1. Field of the Invention
The present invention relates to antenna devices, such as antenna devices preferably for use in a non-contact communication system, for example, a near-field communication (NFC) system, and relates to wireless communication apparatuses including the antenna devices.
2. Description of the Related Art
In recent years, cellular phones and the like each include therein an antenna device used in a non-contact communication system in the 13.56 MHz band, for example. Such an antenna device requires a large coil antenna to obtain a favorable communication range, and the coil antenna is attached to the inner surface of a terminal casing where a relatively large space is available. A feeding circuit (RFIC chip) for processing RF signals is DC-connected to the coil antenna through a connector or pins.
However, in the case of DC connection described above, there is a problem in that contact resistance varies with the roughness of the contact surface, oxidization, and contact pressure, and there is also a reliability problem in that contact failure occurs due to a mechanical shock caused by vibration or dropping.
Hence, it is proposed in Japanese Unexamined Patent Application Publication No. 2008-306689 and Japanese Patent No. 4325621 that a transmission/reception antenna connected to an RFIC chip mounted on a substrate through wiring provided on the substrate and a resonant antenna provided, for example, on the inner surface of a terminal casing are operated in such a manner as to be electromagnetically coupled to each other. According to this proposition, the problems described above are solved and, in addition, the size of the transmission/reception antenna can be reduced since the transmission/reception antenna need only be coupled to the resonant antenna.
However, if the distance between a booster coil antenna and a feeding coil antenna fluctuates, the magnitude of the electromagnetic coupling between the two varies, resulting in a problem in that communication characteristics are degraded since a resonant frequency deviates from a desired value. Further, not all the magnetic fluxes generated by the feeding coil form closed loops. Hence, an increase in the degree of coupling between the two antennas is limited and it is difficult to adjust the degree of coupling to obtain a desired operation frequency.